Cantilever spring seals for gas isolators

ABSTRACT

A seal for a gas isolator comprises a cantilever leaf spring (1) attached to a fixed frame or a movable member (2) of the isolator, and a bias spring (3) acting on the leaf spring, and a bar (4) disposed below the seal for clamping and supporting the latter, characterized by at least one additional damping element which will usually comprise a further leaf spring (7), and characterized in that the clamping/support bar (4) is of reduced width.

This invention relates to seals for gas isolators, and in particular tohigh expansion tolerance seals for large gas isolators where largeamounts of differential expansion have to be accommodated betweencertain components of the isolator, namely, the movable closure member(the blade) and the fixed frame (the casing).

The increasing size of gas turbines is creating a need for largerisolating equipment. One of the main problems which confronts thedesigner of such equipment is that of the differential expansion whichoccurs between the fixed frame and the moving blade member. The amountof expansion depends on the size, the maximum temperature differentialand the materials of construction. For example, on a diverter valve 5.5m square the differential expansion can be as much as 65 mm.

In addition, modern gas turbines have very high exhaust velocities, meanvalues of 80-100 m/s being common. Unless the design of the exhaustducting has been executed using good aerodynamic design principles, flowseparation and high fluctuating dynamic pressures can result. Thefrequencies of the fluctuating pressures can exist over a wide range ofvalues up to as much as 50 hz. Seals for gas isolators located in thesesystems must be designed so that they will not be excited by forcingfrequencies in the system up to say 60 hz. Attention to resistance toflutter is therefore of paramount importance.

In the prior art (GB-1308801 & GB-2060824) the amount of differentialexpansion which can be accommodated is limited by the width of theclamping/support bar under the seal. This bar has the dual function ofclamping the seal to the blade or frame member and to prevent flutter ofthe seal when it is in the non-sealing position.

GB-1308801 describes a seal for a gas isolator which comprises acantilever leaf spring to be attached to a fixed frame or a movablemember of the isolator, and a bias spring acting on the leaf spring totension the leaf spring so as to curve the latter when the seal is notin the sealing position, the bias spring being itself tensioned to pressthe leaf spring flat against a seating when the seal is in the sealingposition.

An alternative form of seal is described in GB-2060824 which provides aseal for a gas isolator which comprises a cantilever leaf spring to beattached to a fixed frame or a movable member of the isolator, and abias spring acting on the leaf spring, wherein the leaf spring is bentor curved when unstressed and wherein in the non-sealing position of theseal the bias spring makes only touch contact with the leaf spring, sothat the leaf spring is unstressed in the said non-sealing position,whereby in the sealing position of the seal the stress induced in thebias spring will be the same as that induced in the leaf spring wherethe leaf spring and the bias spring have the same section modulus perunit length of seal.

A yet further alternative form of seal for a gas isolator is describedin PCT patent application no. PCT/GB89/01382 (publication no. WO90/06460), which provides a seal for a gas isolator which comprises acantilever leaf spring to be attached to a fixed frame or a movablemember of the isolator, and a bias spring acting on the leaf spring,wherein the seal is given an initial curvature corresponding to aninitial deflection of 10% to 90% of the final free deflection,preferably 25% to 75% thereof, more preferably 40% to 60% thereof.

The present invention is particularly concerned with an improvement inor modification of a seal of the type according to GB-2060824, but isalso applicable to a seal of the type according to GB-1308801 orPCT/GB89/01382.

The present invention provides a seal for a gas isolator which comprisesa cantilever leaf spring to be attached to a component of the isolatorselected from a fixed frame or a movable member of the isolator, and abias spring acting on the leaf spring, and a bar disposed below the sealfor clamping and supporting the latter, characterized by at least oneadditional damping element which will usually comprise a further leafspring, and characterized in that the clamping/support bar is of reducedwidth.

The additional damping element will usually comprise a leaf spring ofthe same material and thickness as the cantilever leaf spring, but willusually be somewhat narrower (i.e. shorter) than the seal elementcomprised by the cantilever leaf spring. The additional damping elementwill usually be clamped between the cantilever leaf spring and the fixedframe or movable member, and be given an initial curvature whereinhowever the radius of curvature thereof is less than that of thecantilever leaf spring.

Usually a seal according to the invention will include only a singleadditional damping element, but a corner seal may suitably include two(or more) superimposed additional damping elements in order to counterincreased vibration at the isolator corners, as will be furtherdescribed below.

The present invention thus provides a seal characterized by at least oneadditional damping element and a modified element side clamping/supportbar of reduced width. In this case the damping function is provided byboth the damping element and the clamping/support bar.

The clamping/support bar is reduced in width so that it extends beyondthe edge of the movable member (blade) or frame member to which the sealis clamped by a reduced amount compared with the prior art. In anextreme case it could be reduced to such an extent that it does notprotrude beyond the blade or frame member edge to which it is clamped.The clamping bar is preferably flat in form, and in the case where itextends beyond the blade or frame member to which it is clampedaccordingly produces an interference between it and the sealelement/damping element assembly comprised by the cantilever leaf springand the additional damping spring.

Usually a bias spring clamping bar will be provided to clamp the biasspring, the bias spring clamping bar being located on the opposite sideof the movable member or frame member from the clamping/support bar, andthe clamping/support bar will usually be of a width less than the widthof the bias spring clamping bar.

The damping effect is enhanced by the additional damping element fittedto the same side of the blade or frame member as the sealing elementitself, between the seal element and the blade or frame member. Thisdamping element would be typically but not necessarily of the samethickness as the seal element, and somewhat narrower than the sealelement. By transferring part of the damping function to this dampingelement between the seal element and the blade or frame member allowsthe side clamping/support bar to be reduced in width so that a muchgreater part of the full span width of the seal may be used toaccommodate thermal expansion. Sufficient clearance needs to be providedto take account of manufacturing tolerances in the members.

The seal element is preferably given an initial curvature generally asin the embodiment described in GB-2060824. The additional dampingelement is also given an initial curvature but of a somewhat smallerradius than the seal element. This ensures that, when the assembly ismounted on the blade or frame member, the two seal components form aninterference fit so that there is a mutual contact force between theseal element and the damping element.

The bias spring and a bias spring side clamping bar takes generally theform of that used in the above mentioned prior art. There is a smallcontact force where the bias spring contacts the cantilever leaf springwhen the system is in the free position.

The damping occurs for two reasons. The fact that the free length of thedamping element is not the same as that of the seal element means thatthe natural frequencies of vibration of the two components aredissimilar, so that they cannot vibrate as individual members, but areconstrained to vibrate as a pair having a much higher frequency. Inaddition the interference of the two seal components when assembledcauses a frictional force between the two elements. These mechanismsinhibit the tendency of the seal to flutter.

At a corner seal, two seals respectively extending along perpendicularedges of the fixed frame or movable member are usually mitered togetherat an angle of approximately 45°. In corner seals according to thepresent invention it is preferable to provide two (or more) additionaldamping elements directly superimposed one on the other, in order toreduce or eliminate increased vibration at such corners.

The seal according to the invention may be provided with a second biasspring acting on the cantilever leaf spring, in the manner described inU.S. Pat. No. 5,099,886 corresponding to PCT patent application no.PCT/GB 89/00975 (publication no. WO 90/02279), wherein a second similar,but not identical, bias spring acts on the leaf spring, and in whichpreferably the free ends of the two bias springs act on the leaf springat substantially the same point, and in which each bias spring has afirst limb which is clamped and a second limb which subtends an obtuseangle to the first limb. This will in particular be the case where theseal according to the invention is a corner seal provided with twoadditional damping elements as described above, but a seal of theinvention provided with a second bias spring may also be utilized atnon-corner seals under some circumstances.

Usually the tip of the bias spring forms virtually a line contactbetween itself and the surface of the cantilever leaf spring comprisingthe sealing element. In the event that the assembled seal vibrates,there is a possibility that wear of the sealing element could occur overa narrow band corresponding to the amplitude of vibration.

In an alternative embodiment, therefore, the bias spring may be bent ata location a short distance from the outer end of the bias spring suchthat, in the free position of the assembled seal, a short length of thebias spring lies flat on the cantilever leaf spring. This has the effectthat, for a given contact force, the bearing pressure between the biasspring and the seal surface is much reduced. If any vibration of theassembly occurs the rate of any wear would be much reduced. In amodification of this alternative embodiment, to reduce the possibilityof wear at the seal tip, a short lip may be formed at the outer end ofthe bias spring, such a lip being slightly curved to give a smoothcontact surface between the seal and the bias spring.

The invention will be further described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a side view of a known type of seal, according to GB-2060824,in the free (non-sealing) position thereof;

FIG. 2 is a side view of the seal shown in FIG. 1, in the sealingposition thereof;

FIG. 3 is a side view of a seal according to the present invention, inthe free (non-sealing) position thereof;

FIG. 4 is a side view of the seal shown in FIG. 3, in the sealingposition thereof;

FIG. 5 is an exploded side view of the seal shown in FIGS. 3 and 4;

FIG. 6 is a side view of a seal according to the present invention, inthe free (non-sealing) position thereof, for sealing at a corner;

FIG. 7 is a schematic plan view of the seal according to FIG. 6; and

FIG. 8 is a side view of a further embodiment of a seal according to thepresent invention, in the free (non-sealing) position thereof.

In the drawings, like reference numerals indicate like parts.

The known seal shown in FIGS. 1 and 2 comprises a cantilever leaf spring1 attached to a fixed frame member or to a movable member (blade) 2 ofan isolator, and a bias spring 3 acting on the leaf spring 1. A bar 4 isdisposed below the seal for clamping and supporting the latter, and abias spring clamping bar 5 clamps the bias spring 3 as shown. Theclamping/support bar 4 is in contact with the leaf spring 1 at the pointA as shown in FIG. 1.

Such a seal is of the type according to GB-2060824, wherein in thenon-sealing position of the seal as shown in FIG. 1 the bias spring 3makes only touch contact with the leaf spring 1 so that the leaf springis unstressed in the non-sealing position, while in the sealing positionof the seal as shown in FIG. 2 the stress induced in the bias springwill be the same as that induced in the leaf spring where the leafspring and the bias spring have the same section modulus per unit width.

As shown in FIG. 2, the seal bears against a landing bar (seating) 6which will comprise a movable member or frame member of the isolatoropposite to the frame member or movable member 2. As indicated in FIG.2, the clearance between the bars 4 and 6 will be greater or lesser inthe cold and hot states respectively of the seal. The amount of suchdifferential expansion which can be accommodated and thus limited by thewidth of the clamping/support bar 4.

In the seal according to the invention as shown in FIGS. 3 and 4,wherein like reference numerals indicate like parts as in FIGS. 1 and 2,the side clamping/support bar 4 is reduced in width so that it extendsbeyond the edge of the isolator component, i.e., the blade or framemember 2, to which the seal is clamped by a reduced amount as comparedwith the bar 4 of the known seal of FIGS. 1 and 2. Also, theclamping/support bar 4 will have a width less than the width of the biasspring clamping bar 5.

The bar 4 is flat in form and extends beyond the blade or frame member 2to which it is clamped and accordingly produces an interference at apoint C in FIG. 3 between it and a seal assembly comprised by the sealelement 1 and an additional damping element 7 which is described in moredetail below. Because of the curve of the seal there will be someinteraction between the seal and the bar 4 at the point C, dependingupon the length of the bar 4.

The additional damping element 7 comprises a further leaf spring whichis fitted between the seal element comprised by the cantilever leafspring 1 and the blade or frame member 2, as shown in FIGS. 3 and 4. Theadditional damping element 7 is preferably of the same thickness as theseal element 1, and is narrower (shorter) than the seal element. Byvirtue of the enhanced damping effect produced by the element 7, theclamping/support bar 4 can be reduced in width as compared to the knownstructure of FIGS. 1 and 2, so that a much greater part of the full spanwidth of the seal can be used to accommodate thermal expansion.

The seal element comprised by the cantilever leaf spring in theembodiment of FIGS. 3 and 4 is given an initial curvature generally asin the prior art structure according to GB-2060824 and shown in FIGS. 1and 2.

The additional damping element comprised by the leaf spring 7 is alsogiven an initial curvature but of somewhat smaller radius than the sealelement 1. As shown more particularly in FIG. 5, the free radius ofcurvature R₄ of the damping element 7 is less than the free radius ofcurvature R₅ of the seal element 1. This ensures that when the sealassembly is mounted on the blade or frame member then the two components1 and 7 form an interference fit so that there is a mutual contact forcebetween the seal element 1 and the damping element 7 at a point B wherethe free end of the damping element 7 bears against the seal element 1(see FIG. 3). That is, the seal element 1 and the damping element 7 willbe equally stressed, where they are of the same material and have thesame thickness.

The bias spring 3 and the bias spring clamping bar 5 are generally as inthe known construction shown in FIGS. 1 and 2. There is a small contactforce at a point D where the bias spring 3 contacts the seal elementcomprised by the cantilever leaf spring 1, when the seal is in the freeposition.

As described previously, the damping occurs for two reasons. Firstly,the fact that the free length of the damping element 7 is not the sameas that of the seal element 1 means that the natural frequencies ofvibration of the two components are dissimilar, so that they cannotvibrate as individual members, but are constrained to vibrate as a pairhaving a much higher frequency. In addition, the interference of the twoseal components when assembled causes a frictional force between the twocomponents. These mechanisms inhibit the tendency of the seal toflutter.

The corner seal shown in FIG. 6 is generally similar to that shown inFIGS. 3 and 4, except that the single additional damping element 7 ofFIGS. 3 and 4 is replaced by two superimposed additional dampingelements 7a and 7b in the embodiment of FIG. 6, which will usuallycomprise leaf springs as previously described. The two damping elements7a and 7b are essentially identical and function in a similar manner tothe single damping element 7 of the embodiment of FIGS. 3 and 4, butcounter the increased vibration which is liable to occur at a cornerseal. It has been determined that, by using a corner seal as shown inFIG. 6, no vibration was found to occur up to 100 hz. Although FIG. 6shows the free ends of the damping elements 7a and 7b acting atsubstantially the same point, it is alternatively possible to providedamping elements of different length, wherein the end of a shorterdamping element 7b acts on a longer damping element 7a.

FIG. 7 shows schematically a corner seal mitered (at the left hand sideof the drawing) at an angle of approximately 45° to fit against asimilar corner seal (not shown) extending along a perpendicular edge ofthe blade or frame member on which the seals are carried. Referencenumeral 8 indicates the overlap by the cantilever leaf spring 1 on alanding bar (seating), while reference numeral 9 indicates the area ofoverlap of the additional damping elements 7a and 7b on the leafspring 1. The bias spring 3 is omitted from FIG. 7 for reasons ofclarity.

The corner seal described above with reference to FIGS. 6 and 7 ispreferably (but not necessarily) provided with a second bias spring (notshown) acting on the cantilever leaf spring 1, in the manner describedin the above mentioned U.S. Pat. No. 5,099,886 corresponding to PCTpatent application no. PCT/GB 89/00975 (publications no. WO 90/02279),wherein a second similar, but not identical, bias spring acts on theleaf spring, and in which preferably the free ends of the two biassprings act on the leaf spring at substantially the same point, and inwhich each bias spring has a first limb which is clamped and a secondlimb which subtends an obtuse angle to the first limb.

A seal of the invention provided with such a second bias spring may alsobe utilized at non-corner seals under some circumstances.

In the various embodiments described above, the tip of the bias spring 3forms virtually a line contact between itself and the surface of thecantilever leaf spring 1 comprising the sealing element. In the eventthat the assembled seal vibrates, there is a possibility that wear ofthe sealing element could occur over a narrow band corresponding to theamplitude of vibration.

In an alternative embodiment, therefore, as shown in FIG. 8, the biasspring 3 is bent at a location 3a a short distance from the outer endthereof such that, in the free position of the assembled seal, a shortlength x of the bias spring lies flat on the cantilever leaf spring 1.This has the effect that, for a given contact force, the bearingpressure between the bias spring and the seal surface is much reduced.If any vibration of the assembly occurs the rate of wear would be muchreduced.

I claim:
 1. A seal for a gas isolator, comprising a cantilever leafspring to be attached to an isolator component selected from a fixedframe or a movable member of the isolator, a bias spring acting on thecantilever leaf spring, a clamping/support bar disposed below the sealfor clamping and supporting the seal, at least one additional dampingelement which is clamped between the cantilever leaf spring and saidisolator component, and a bias spring clamping bar to clamp the biasspring, said bias spring clamping bar being located on the opposite sideof said isolator component from the clamping/support bar, and theclamping/support bar being of a width not greater than the width of thebias spring clamping bar.
 2. A seal as claimed in claim 1, wherein saidadditional damping element comprises a further leaf spring.
 3. A seal asclaimed in claim 1 or 2, wherein said additional damping elementcomprises a further leaf spring of the same material and thickness asthe cantilever leaf spring, and said further leaf spring has a widthless than that of the cantilever leaf spring.
 4. A seal as claimed inclaim 1 or 2, wherein said additional damping element has an initialcurvature the radius of curvature (R₄) of which is less than the radiusof curvature (R₅) of the cantilever leaf spring.
 5. A seal as claimed inclaim 1 or 2, wherein the clamping/support bar extends beyond the edgeof said isolator component.
 6. A seal as claimed in claim 1 or 2,wherein said at least one additional damping element comprises twosuperimposed leaf springs.
 7. A seal as claimed in claim 1 or 2, whereinthe bias spring is bent adjacent the free end thereof so that, in thefree position of the assembled seal, the end portion of the bias springlies flat on the cantilever leaf spring.